In recent years, porous filtration membranes such as ultrafiltration membranes and microfiltration membranes have been increasingly used in various industrial fields including the field of water treatment associated with the production of drinking water, clean water/waste water treatment, hemocatharsis and other medical applications, and the food industry, and the field of organic solvent treatment associated with the refinement of waste oil, the production of bio-ethanol and the filtration of chemical agents in the semiconductor industry. To this end, porous filtration membranes having various pore size have been developed. Particularly, common porous filtration membranes having pore size on the order of nm to μm are generally produced by the phase separation of an organic polymer solution. This process is applicable to various organic polymer compounds to permit easy industrialization and, therefore, has become a predominant process for industrially producing the filtration membranes.
The phase separation process generally includes a non-solvent induced phase separation process (NIPS process) and a thermally induced phase separation process (TIPS process). In the NIPS process, a homogeneous polymer solution experiences phase separation due to a change in concentration caused by ingress of a non-solvent or evaporation of the solvent to the outside atmosphere. On the other hand, the TIPS process is a relatively new process, in which a homogeneous polymer solution prepared by dissolution of a polymer at a higher temperature is cooled to a temperature below a binodal curve (a boundary line between a first phase and a second phase) to induce the phase separation and the resulting structure is fixed by the crystallization or glass transition of the polymer.
Exemplary materials for the porous filtration membranes to be often used in the known art include polyolefins such as polyethylenes and polypropylenes, polyvinylidene fluorides, polysulfones, polyether sulfones, polyacrylonitriles and cellulose acetates. However, the polyolefins, the polyvinylidene fluorides, the polysulfones, the polyether sulfones and the like are highly hydrophobic. Where a porous filtration membrane is formed from any of these materials, therefore, the porous filtration membrane disadvantageously has a lower water flow rate, and is liable to be fouled to have a reduced water permeability because of its nature of adsorbing hydrophobic substances such as proteins. The polyacrylonitriles, the cellulose acetates and the like are more hydrophilic resins, but a membrane formed from any of these resins is poor in strength and less resistant to temperature and chemicals. Therefore, the membrane is usable in a very narrow temperature range and in a very narrow pH range. The polyvinylidene fluorides, the polysulfones, the polyether sulfones and the like are less resistant to organic solvents and, therefore, cannot be used for filtration of the organic solvents.
Therefore, a method of producing a porous membrane from a polyamide resin which is relatively highly hydrophilic and highly resistant to chemicals is contemplated. However, the polyamide resin is soluble only in a strong acid such as formic acid or concentrated sulfuric acid or in an expensive fluorine-containing solvent. Therefore, there is no other way but to use any of these solvents for the production method employing the NIPS process.
Membrane production methods employing formic acid as the solvent are disclosed, for example, in JP57-105212A (1982), JP58-065009A (1983), U.S. Pat. No. 4,340,479B and U.S. Pat. No. 4,477,598B. However, these membrane production methods are likely to pose health and safety problems. JP2000-001612A discloses a method which includes the steps of dissolving polyamide 6 mixed with polycaprolactone in hexafluoroisopropanol, casting the resulting solution, and extracting polycaprolactone from the resulting product to produce a porous product. However, this method is impractical, because the solvent to be used and the polymer to be extracted off are highly expensive.
On the other hand, a method employing the TIPS process is also contemplated. Journal of Membrane Science 108, pp 219-229 (1995) states that a porous membrane can be produced by employing polyamide 12 and polyethylene glycol in combination. U.S. Pat. No. 4,247,498B states that a porous membrane can be produced by employing polyamide 11 and one of ethylene carbonate, propylene carbonate and sulfolane in combination. Membrane Technology 2nd Edition, authored by Marcel Mulder under the supervision of Masakazu Yoshikawa, Takeshi Matsuura and Tsutomu Nakagawa and published by IPC, pp 95 (1997), states that a porous membrane of polyamide 6 and polyamide 12 can be produced by employing triethylene glycol as a solvent. However, these methods merely permit the production of porous membranes, but fail to impart the hollow fiber membranes with a higher water permeability and to control the pore size of the membranes.
As described above, the polyamides can be used for producing a hollow fiber membrane through the NIPS process employing formic acid and for producing a porous membrane through the TIPS process. However, it is very difficult to produce a hollow fiber membrane through the TIPS process, because the solution resulting from the higher temperature dissolution has a lower viscosity and a lower specific gravity. Therefore, there is virtually no successful example. JP58-164622A, for example, discloses porous membranes formed from various types of resins through the TIPS process and examples of polyamide flat membranes, but provides no teaching on production of a polyamide hollow fiber membrane. JP60-052612 discloses a method of producing a hollow fiber membrane through the TIPS process. However, the resulting hollow fiber membrane has a very great pore size, i.e., 1.4 μm and, therefore, cannot be used for filtration processes intended by the present invention, i.e., for ordinary water treatment, blood treatment and filtration processes in the food industry and the pharmaceutical industry. JP60-052612 states that a membrane can be produced by using glycerin or ethylene glycol. However, the membrane thus produced fails to have sufficient strength for practical use. In U.S. Pat. No. 4,915,886B, an apparatus of producing a polyamide hollow fiber membrane through the TIPS process is described in detail. However, no detailed description is given to the type of the polyamide and a usable solvent, and no inventive example is provided. Even though permitting the production of the hollow fiber membrane, U.S. Pat. No. 4,915,886B does not teach the formulation and the properties of the resin. JP2003-534908A (WO01/093994) discloses a method which involves addition of an antioxidant for suppression of decomposition of a polyamide membrane. However, a solvent used in this method is expensive. JP2003-534908A states that a hollow fiber membrane production method of an inventive example involves addition of a thickener, and that the resulting hollow fiber membrane has a greater pore size, i.e., a maximum pore size of 0.87 μm and 0.57 μm, but provides no other detailed description.
As described above, a technique related to a polyamide hollow fiber membrane having a higher water permeability and a higher particle rejection percentage is not known yet.